Tutorial:
Make a File in your C Drive (C and call it "steam" and the go inside of that folder and make another one called "counterstrikesource"
You will then need to open SteamCMD and Type: Login Anomoymous

Then Type this: force_install_dir C:\steam\counterstrikesource
( You will not get any identification on screen that it worked, all that does is select a area to install the update)
Then Type: app_update 232330 validate

It will then need to download for about 20 mins with a good internet.
When it finishes you can close it down and go back to that steam folder we created earlier and enter the counterstrikesource folder.

Now you will see a file called cstrike, right click on it and copy it.
Then go to your server file where your garrysmod and srcds.exe is located and drop it in there.
( Do NOT drop inside the garrysmod folder only next to it. )

Now go into the garrysmod folder and go cfg and inside you will see mount.cfg. ( If you dont have one, make a text file. Open it and press Save as and name it mount.cfg. )
Copy and Paste this code:
"mountcfg"
{
"cstrike" "Your cstrike location in your game server goes here"
// "tf" "C:\mytf2server\tf"
}

Example: ( This is mine but yours can be different but must have the same concept. )
"mountcfg"
{
"cstrike" "C:\GMOD\cstrike"
// "tf" "C:\mytf2server\tf"
}

It might take a few restarts (It did for me ) to get the server to boot with the game mounted as you will get this line of code in your srcds.exe:
Adding Path: [C:\GMOD\cstrike]
FIND [C:\GMOD\cstrike\*.vpk]
ADDING [C:\GMOD\cstrike\cstrike_pak_000.vpk]